The present invention relates generally to tilting micromechanisms, and more specifically to a new class of micromechanisms able to attain large tilt angles owing to the use of multiple floating pivot structures.
Although there is no precise referent for the term "micromechanical", its common use is to machinery whose size scale makes invalid most of our assumptions about machinery of ordinary dimensions. For example, whereas at ordinary dimensions the strength of materials is usually limited by mechanisms involving motion and multiplication of dislocations in response to applied stress, in the micromechanical regime surface erosion and cleavage modes of failure are more common. Again, whereas at ordinary dimensions fluid lubrication of frictionally related elements (e.g., an axle and a shaft bearing) is one of a handful of practical choices, in the micromechanical regime the viscous shear stress of a lubricant increases in inverse proportion to the size scale of the apparatus, increasing the energy dissipation rate to unusable levels.
When the change in size scale renders conventional practice invalid, it also enables new approaches toward micromechanical functionality. An effective replacement for fluid lubricants can be a surface layer of a solid which is (usually) either amorphous or polycrystalline, is reasonably strong, forms a smooth film on the component surfaces, and has a surface structure that resists bonding with itself.
Consistent with common practice, in this specification the term `micromechanical` is associated with mechanical apparatus whose functional components have sizes ranging from about 1 mm to about 1 .mu.m. Such micromechanical apparatus can be made of a wide variety of materials, but perhaps the most common system is a combination of polycrystalline silicon, amorphous silicon, silicon oxides, and silicon nitride. This material system has high material strength and rigidity, high resistance to fracture, can be doped to provide electrically conducting regions, and is relatively easy to directly integrate with microelectronics at a chip level.
A common mechanism in micromechanical apparatus is a tilt stage, whose function is to provide an attached element with a controllable amount of tilt along a predetermined axis or axes. The attached element can be an intrinsic part of the apparatus (e.g., as in an escapement), or an external element which is to be positioned and directed by the apparatus (e.g., a micromirror). The implementations described in this specification will be for micromirror tilt stages, but this should not be taken as an implicit limitation on the scope of the present invention.
A typical prior art micromirror tilt stage is shown in FIGS. 1a-1d. FIG. 1a shows a transparent top view of the tilt stage, and FIG. 1b shows a side view. Micromirror 11 is mounted on top of base 10 by flexible pivot 12. (Note that 11 could as easily be a mounting platform for a micromirror or for some other component. The present identification is made for simplicity of description, and not to limit the scope of the present invention.) The top surface of micromirror 11, if necessary, is polished flat and/or coated with a reflective layer.
Capacitor plates 13 and 14 provide the forces which tilt the micromirror. In one working arrangement the micromirror is doped to the point of being a good conductor, and is grounded through flexible pivot 12 and base 10, both of which are also appropriately doped. Capacitor plates 13 and 14 are electrically insulated from the base 10. When a voltage is applied to capacitor plate 13, the micromirror 11 tilts to the left. When a voltage is applied to capacitor plate 14, the micromirror 11 tilts to the right.
If the applied voltage is large enough, the micromirror 11 will tilt until it hits a solid stop. This condition is of interest for designers, as it provides for a precise amount of tilt, and at the same time helps prevent vibration of the micromirror. Two common solid stops are illustrated here. FIG. 1c shows a maximum leftward tilt, the magnitude of the tilt being limited by contact between the micromirror 11 and the base 10. This provides a limitation in many designs, because the micromirror structure is fabricated by growing layers of structural and sacrificial material on top of the base, and then removing the sacrificial material. Practical limitations of such processes limit the thickness of the sacrificial layers to a few microns at most. A typical micromirror can be perhaps 100 .mu.m across, with a gap separating the micromirror from the base of about 1 .mu.m. In this case, the maximum tilt angle is limited to about 1.degree..
FIG. 1d shows a maximal rightward tilt, where the magnitude of the tilt is limited by the presence of stop post 15. Such a stop post can be used if a smaller maximum tilt is desired than results from contact between the micromirror and the base.
The tilt stage described above allows tilt along arbitrary directions. This is desirable for some applications, and not desirable for others. A related tilt stage with an additional torsional pivot can be arranged, as shown in FIG. 2, to define a single restricted tilt axis. Within this specification and the attached claims, a tilt stage has a restricted tilt axis (or multiple restricted tilt axes) only when the structure of the tilt stage substantially restricts the tilting motions of the tilt stage to those corresponding to that restricted tilt axis (or to those restricted tilt axes). Equivalently, the amount of force which must be supplied in the initial stages of tilting about a restricted tilt axis is a local minimum.
In this case of FIG. 2, tilting around the axis defined a line between the two torsional pivots and parallel to the base surface is much easier than in any other direction. This axis is therefore a restricted tilt axis.
In another implementation of this prior art device, a second pair of capacitor plates can be added to the base, thereby allowing an arbitrary tilt direction to be driven electrostatically, as shown in FIG. 3.
Many equivalent mechanisms are known in the art. In particular, whereas the pivoting motion was provided above by the bending of a flexible pivot connecting the micromirror to the base, many other structures produce equivalent devices. For example, the micromirror can be attached by a pair of torsional pivots attached to the sides of the micromirror and to a mounting frame which is attached to the base. The top surface of a cantilever beam can be used as a micromirror or as a micromirror mount. Such a cantilever mount can provide multiaxis tit capability when electrostatic actuators are arranged so that both bending of the cantilever and rotation about the cantilever are driven.
Many applications for tilt stages, especially those involving the use of micromirror arrays in active optical processing or display applications, require or can beneficially use larger tilt angles than are accessible using the basic prior art mechanisms described above. However, the process-related limitation to the extent of tilt cannot easily be circumvented, particularly when an array of closely spaced micromirrors is the desired product.
Applicants know of only two prior art tilt stage designs which relieve this tilt angle limitation. Both are limited in mechanical generality and in range of application.
First to be developed was a tilt stage with an eccentrically placed flexible pivot, as shown in FIG. 4a and 4b. This tilt stage is rather similar to that of FIG. 1, having a micromirror 41 attached to a base 40 by a flexible pivot 42, and a capacitor plate 43 to drive motion of the tilt stage when an appropriate potential difference is applied. However, rather than the centrally located flexible pivot of FIG. 1, this flexible pivot is offset toward the capacitor plate. As a result, the maximum tilt angle toward the left (FIG. 4c) is increased by the appropriate geometrical factor, which in practice can be as large as 3-5 times.
This increased tilt angle comes with two costs, however. First, the force provided by the capacitor plate 43 must be considerably larger than that required by a tilt stage after FIG. 1 to obtain the same responsiveness. Second, the increased tilt is only available in one direction--the maximum rightward tilt for this configuration is typically about half that obtained with a central flexible pivot. These limitations combined limit this type of prior art tilt stage to a small number of applications.
The second prior art mechanism which achieves large tilt angles was also developed here at Sandia National Laboratories, and comprises a combination of hinges, sliding bearings, and linear drive mechanisms. Such a mechanism is shown in FIG. 5. Assembled from carefully patterned material and sacrificial layers on a base 500, a micromirror 504 is mounted on top of a mirror frame 503. Mirror frame 503 is rotably fixed to base 500 by a first set of axles 502 attached to mirror frame 503 and rotating in a first pair of bearing blocks 501 which are at the surface of base 500.
Mirror frame 503 is rotably fixed to driving frame 507 by a second set of axles 505 attached to mirror frame 503 and rotating in a second pair of bearing blocks 506 which are attached to driving frame 507. Driving frame 507 is secured to base 500 by a third pair of axles 509 which are attached to the driving frame, and are constrained to slide on the surface of the base by a pair of sliding bearings 508.
Driving frame 507 is moved along the surface of the base by the action of linear transfer beam 513, which transfers motion and force from a bi-directional linear electrostatic actuator comprising comb electrodes 514, 515, 517, and 518. The linear transfer beam 513 is restricted to move substantially along its long axis through the combined action of support bushing 516, beam guides 512, and rotable connection 510 and 511 to the driving frame.
The function of this prior art tilt stage can be understood by comparing the side view of FIG. 5a with FIG. 5b. FIG. 5a shows the `flat` configuration, in which the electrostatic actuator has pulled linear transfer beam 513 as far to the right as is possible. In the flat configuration the micromirror 504 is in a well-defined position and orientation approximately parallel to the surface of base 500. The exact position can be adjusted in the design phase by placing additional material under either between the micromirror and the mirror frame, or by placing additional material under the mirror frame.
FIG. 5b shows the `tilted` configuration in which the electrostatic actuator has pushed the linear transfer beam 513 as far to the right as possible. In the tilted configuration micromirror 504 achieves a well-defined position which is tilted at a large angle relative to the surface of base 500.
The device of FIG. 5 offers a number of advantages over the prior art flexible pivot tilt stages. The transition between the flat and the tilted configurations is rapid and energy efficient, and the maximum attainable angle can be as large as 60.degree.. However, the apparatus allows tilt only in one direction, and the mechanism is extremely complex. A related mechanism can be designed which allows bi-directional tilt, but the complexity of the tilt stage increases dramatically. In addition, the design uses a very large amount of surface area relative to the size of the micromirror. Finally, there is considerable looseness in most of the moving joints and connections in the tilt stage, partially because of the basic design and partially because of the tolerances achievable for such joints using current micromechanical fabrication techniques. As a result, this type of prior art tilt stage is only occasionally used for special applications well-suited to its characteristics.
A simple, easily constructed tilt stage capable of large tilt angles, preferably being capable of such action about multiple tilt axes, is badly needed in this realm of the useful arts.